Complications such as restenosis are a recurring problem in patients who have received artherosclerosis therapy in the form of medical procedures such as percutaneous transluminal coronary angioplasty (PTCA). Restenosis is commonly treated by a procedure known as stenting, where a medical device is surgically implanted in the affected artery to prevent it from occluding post procedure.
A stent is typically cylindrical in shape and is usually made from a biocompatible metal, such as titanium or surgical steel. Most stents are collapsible and are delivered to the occluded artery via transluminal catheter. The stent is affixed to the catheter and can be either self expanding or expanded by inflation of a balloon inside the stent that is then removed with the catheter once the device is in place. Examples of common types of stents are disclosed in US Patent Application Publication No. U.S. Pat. No. 6,936,066 to Palmaz entitled “Complaint Implantable Medical Devices and Methods of Making Same.”
Complications that arise from stent therapy are restenosis and thrombosis. In an effort to overcome these complications, stents are routinely developed to have the additional feature of controlled drug elution. To accomplish this, a metal stent is coated with an API mixed with polymer, as disclosed in U.S. Pat. No. 5,716,981 issued to Hunter entitled anti-angiogenic Compositions and Methods of Use. Examples of typical therapies that are delivered in this manner are antiproliferatives, anticoagulants, anti-inflammatory agents and immunosuppressive agents, though there are many other chemical and biological agents that can be used. A porous layer of biodegradable material is often applied over the coating layer to regulate controlled release of the drug into the body. Common types of polymer coated drug eluting stents are disclosed by U.S. Pat. Nos. 6,774,278 and 6,730,064 issued to Ragheb entitled “Coated Implantable Medical Device.”
It has been postulated that the presence of this polymer contributes to thrombosis due to delamination. It is thought that over time, the protective polymer may separate from the bare metal or substrate, resulting in sharp peaks or edges that come in direct contact with red blood cells, the result of which is thrombosis, causing serious illness or death in the patient. Stents have been designed that do not include a permanent polymer, like BSI biodegradable PLLA. Some stent designs have moved towards the polymerless altogether such as surface textured stents or biocompatible oil coatings.
To increase the drug loading capacity, stents can be engineered to have roughened surfaces. Rough surfaces on the stent provide for peaks and valleys which increase the total surface area of the stent thereby increasing the amount of API that may be associated with the stent. Roughening of the surface of a stent is accomplished in a number of ways, such as sintering, as disclosed in U.S. Pat. No. 5,843,172 issued to Yan entitled “Porous Medical Stent.” Surface roughness is also achieved by abrasive techniques such as sandblasting and reductive acid etching as disclosed in European patent No. 0806212 issued to Leitao entitled “Device For Incorporation and Release of Biologically Active Agents.” Roughening of a stent surface can also be achieved pressing indentations directly onto the device as disclosed in U.S. Pat. No. 7,055,237 Issued to Thomas entitled “Method of Forming a Drug Eluting Stent” or using the common metal working techniques of shot peening or laser peening. A stent having a mechanical anchoring layer is described in co-owned U.S. Publication No. 2006/0069427. However, the stent and catheter crossing profile serve as physical limitations for the thickness of coatings. A problem encountered with rough stents is that the crossing profile of the stent and catheter must be limited to a thickness that is narrow enough to pass through an occluded artery.
In light of the complications associated with stent therapy, it would be desirable to develop a stent that will have the increased surface area of a rough stent which can be manufactured in such a way as to maximize structural integrity and drug loading capacity. Further, it would be desirable to develop a polymerless stent that is capable of delivering API to decrease or eliminate the risk the risk of late stent thrombosis. Finally, it is desirable to develop a stent that maximizes drug loading capacity while at the same time minimizing the total thickness of the stent-catheter crossing profile.